Durability Enhanced And Redundant Embedded Sensors

ABSTRACT

A substantially planar eddy-current sensor having durability enhancing pillars in an active region is provided. The pillars are distributed and sized so as to have limited effect on the sensor&#39;s performance. When the sensor is mounted on a component such that the sensor experiences forces on a top and bottom surface, the pillars bear the load reducing the load bore by the active elements (e.g., drive winding, sense elements). A sensor with redundant drive windings and/or redundant sense elements is disclosed. The redundant elements may be connected to separate electronics. Another aspect relates to providing a reference transformer for calibration of a sensor. The secondary windings of the reference transformer are connected in series with the sense elements of the sensor to be calibrated. Transimpedance measurements are made when the drive winding of the reference transformer is excited. The measurements are used to correct transimpedance measurements made when the drive winding of the sensor is excited. A system having an impedance analyzer and a plurality of multiplexing units is disclosed for monitoring a plurality of sensor. Each multiplexing units directs an excitation signal to the drive winding of a respective sensor and returns, serially, the sense element responses back to the impedance analyzer. The system coordinates the excitation of each sensor and return of the sensor response to share a serial network. The multiplexing units may have a reference transformer for calibration of their respective sensors. Optical communication may be used.

RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 13/024,154filed on Feb. 9, 2011, which claims the benefit of U.S. ProvisionalApplication No. 61/303,033, filed on Feb. 10, 2010, which are hereinincorporated by reference in their entireties.

GOVERNMENT SUPPORT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractnumber FA8501-09-P-0123 awarded by the Air Force.

BACKGROUND OF THE INVENTION

Delayed detection of cracks in a component adds considerably to the costof repair of products in the aerospace and other industries. Under somecircumstances early detection of a crack may enables not only avoidanceof catastrophic failure but also the ability to repair the component,thus increasing the life of the component and delaying the component'sreplacement.

Cracks are likely to form around high stress areas of a component suchas near or at fastener holes. If a crack near a fastener hole isdetected early, the hole may be oversized to eliminate the crack andextend the life of the component.

Conventional non-destructive testing (NDT) techniques for inspection atfastener holes have had difficulty detecting cracks early enough so thatrepair is possible. Accordingly the component may have to be replaced atconsiderable expense. Ultrasonic testing (UT) which detects reflectionsof a sound wave from within the component under test have proven to havedifficulty detecting cracks beyond the first layer of a component,particularly when the sealant is not uniform or geometric variationsreduce the probability of detection (POD) significantly below 1.0.

Conventional eddy current sensing involves the excitation of aconducting winding (the primary) with an electric current source ofprescribed frequency. The current in the conducting winding produces atime varying magnetic field at the same frequency. By Faraday's law ofinduction an electromotive force is induced in a sensing winding (thesecondary). The electromotive force may then be measured as a voltage.The spatial distribution of the magnetic field which is measured by thesecondary is influenced by the proximity and physical properties (e.g.,conductivity and permeability) of nearby materials. When the sensor isintentionally placed in close proximity to a test material, the physicalproperties of the material can sometimes be deduced from measurements ofthe transimpedance between the primary and secondary windings.Traditionally, scanning of eddy current sensors across the materialsurface is then used to detect flaws, such as cracks. Conventional eddycurrent testing (ET) has proven inadequate for detection of cracks nearfastener holes in multiple layered structures. This is due to difficultypenetrating through thick outer layers and due to noise caused bygeometric variations, fastener fit, and crack morphology variations.

SUMMARY OF THE INVENTION

A substantially planar eddy-current sensor is disclosed that hasdurability enhancing pillars in an active region. The pillars aredistributed and sized so as to have limited effect on the sensor'sperformance. When the sensor is mounted on a component such that thesensor experiences forces on a top and bottom surface, the pillars bearthe load reducing the load bore by the active elements (e.g., drivewinding, sense elements). A sensor with redundant drive windings and/orredundant sense elements is disclosed. The redundant elements may beconnected to separate electronics.

Another aspect relates to providing a reference transformer forcalibration of a sensor. The secondary windings of the referencetransformer are connected in series with the sense elements of thesensor to be calibrated. Transimpedance measurements are made when thedrive winding of the reference transformer is excited. The measurementsare used to correct transimpedance measurements made when the drivewinding of the sensor is excited.

A system having an impedance analyzer and a plurality of multiplexingunits is disclosed for monitoring a plurality of sensor. Eachmultiplexing units directs an excitation signal to the drive winding ofa respective sensor and returns, serially, the sense element responsesback to the impedance analyzer. The system coordinates the excitation ofeach sensor and return of the sensor response to share a serial network.The multiplexing units may have a reference transformer for calibrationof their respective sensors. Optical communication may be used.

Some aspects relate to a substantially planar sensor having a firstsurface and a second surface. The sensor comprises a drive winding, asense winding, a plurality of pillars and a substrate. The drive windingis configured to guide an electrical current. The sense elementconfigured to couple to a magnetic field produced if the drive windingis excited with the electric current. The plurality of pillarsconfigured to protect the drive winding and sense element if the firstand second surfaces are under load. The substrate configured to providemechanical support to the drive winding, the sense element and thepillars.

In some embodiments the drive winding has a first dimension in adirection perpendicular to the first surface of the sensor; the senseelement has a second dimension in the direction perpendicular to thefirst surface of the sensor, and each of the plurality of pillars has athird dimension in the direction perpendicular to the first surface ofthe sensor, said third dimension being greater than the first dimensionand greater than the second dimension.

In some embodiments, the drive winding comprises a single turn.

In some embodiments, the sensor further comprises a redundant drivewinding. The substrate may be formed from a plurality of layers and thedrive winding and the redundant drive winding are formed betweendifferent layers of the substrate.

Some aspects relate to a sensor where the sense element is among aplurality of sense elements that are each configured to couple to themagnetic field if the drive winding is excited with the electricalcurrent.

Some aspects relate to a sensor where the drive winding comprises asubstantially circular portion and the substrate has a hole in a regioncomprising the center of the substantially circular portion of the drivewinding.

Some aspects relate to a sensor where the drive winding is a first drivewinding and the sensor further comprises a second drive winding having asubstantially circular portion of a different radius than thesubstantially circular portion of the first drive winding.

Another aspect relates to a method of using a sensor to detect damagenear a fastener hole of a component. The method comprises mounting thesensor at the fastener hole location of the component; driving the sensewinding to produce the electric current; monitoring a response of thesense element to the electric current; and determining a damagecondition of the component from the response.

Some aspects relate to a eddy-current sensor having a substantiallyplanar surface. The sensor comprises a drive winding, a sense element, aplurality of pillars and a substrate. The plurality of pillars eachpillar comprising a plurality of pillar elements that are aligned in adirection perpendicular to the substantially planar surface. Thesubstrate configured to provide mechanical support to the drive winding,the sense element and the plurality of pillars.

Another aspect relates to a substantially planar eddy current sensorhaving a first surface and a second surface. The sensor has a firstdrive winding, a second drive winding, a plurality of sense elements,and a substrate. The first drive winding configured to guide a firstelectrical current, the first drive winding comprising a substantiallycircular portion having a first radius. The second drive windingconfigured to guide a second electrical current, the second drivewinding comprising a substantially circular portion having a secondradius greater than the first radius. The plurality of sense elementsarranged about a perimeter of the substantially circular portion of thefirst drive winding, each sense element configured to couple to at leastone of a magnetic field produced by the first electric current and amagnetic field produced by the second electric current. The substratematerial configured to provide mechanical support to the first drivewinding, the second drive winding and the plurality of sense elements.

In some embodiments, the sensor further comprises a plurality of pillarsconfigured to protect the drive winding and sense element if the firstand second surfaces are under load. The first drive winding maycomprises a single, substantially circular portion having a first radiusand wherein the sensor further comprises a substantially circular holeconcentric with the substantially circular portion of the first drivewinding, the hole having a second radius less than the first radius.

Some aspects relate to a sensor where the plurality of sense elementsare arranged outside the perimeter of the substantially circular portionof the first drive winding and the second drive winding is so positionedsuch that the plurality of sense elements are arranged inside aperimeter of the substantially circular portion of the second drivewinding. The first drive winding and the second drive winding may beconnected in series.

Some aspects relate to a sensor where the plurality of sense elementsare arranged outside the perimeter of the substantially circular portionof the first drive winding and the second drive winding is so positionedsuch that the plurality of sense elements are also arranged outside aperimeter of the substantially circular portion of the second drivewinding.

Some aspects relate to a sensor where the substrate comprises aplurality of layers, the first drive winding is formed on a first layersurface and the second drive winding is formed on a second layersurface.

Some aspects relate to a sensor where the first layer surface and thesecond layer surface are opposite sides of a same layer among theplurality of layers of the substrate.

Some aspects relate to a sensor where the plurality of sense elementscomprises a first subset of sense elements formed on the first layersurface, and a second subset of sense elements formed on the secondlayer surface.

Another aspect relates to a system for calibrating an eddy-currentsensor. The system comprises a reference transformer, a source, andimpedance analyzer and a processor. The reference transformer having aprimary winding, a reference winding and a secondary winding, whereinthe secondary winding is configured to be connected in series with asense element of the eddy-current sensor. The source for providing adrive signal to the primary winding. The impedance analyzer configuredto measure a first transimpedance of the series connected secondarywinding and the sense element while the primary winding is excited bythe source, and further configured to measure a second transimpedance ofthe series connected secondary winding and sense element while a drivewinding of the sensor is excited. The processor configured to compute acalibrated transimpedance measurement using the first transimpedance andthe second impedance.

Yet another aspect relates to a sensor monitoring system comprising aplurality of eddy-current sensors, an impedance analyzer and a pluralityof multiplexing units. The plurality of eddy-current sensors, eachsensor having a drive winding and a plurality of sense elements. Theimpedance analyzer configured to generate a control signal and anexcitation waveform, and to measure a response waveform. The pluralityof multiplexing units that are each connected in series with one anotherand the impedance analyzer and are each connected to a respective sensoramong the plurality of sensors, each of the plurality of multiplexingunits configured to provide the excitation waveform to the drive windingof the respective sensor in response to a trigger in the control signaland to sequentially pass each of the plurality of sense elementresponses to the impedance analyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1A is a system for inspecting a component under test according tosome embodiments;

FIG. 1B is a cross section of the component under test and a sensoradapted for inspection near a fastener hole according to someembodiments;

FIG. 2A is a depiction of the active area of a sensor having durabilityenhancing pillars according to some embodiments;

FIG. 2B is a cross section of a sensor having durability enhancingpillars according to some embodiments;

FIG. 3A is a schematic of a sensor having durability enhancing pillarsadapted for inspection near a fastener hole according to someembodiments;

FIG. 3B is a detailed drawing of the active area of the sensor adaptedfor inspection near a fastener hole having durability enhancing pillarsaccording to some embodiments;

FIG. 3C is a rendering illustrating mounting of a fastener hole sensorat a fastener hole according to some embodiments;

FIG. 3D shows a fastener hole sensor mounted at a fastener hold and heldby a fastener according to some embodiments;

FIG. 4A is an illustration of the active area of a sensor havingdurability enhancing pillars and an inner and outer drive windingaccording to some embodiments;

FIG. 4B is a cross section of a sensor having two drive windings and adurability enhancing pillars according to some embodiments;

FIG. 5 shows a sensor having durability enhancing pillars and an innerand outer drive winding according to some embodiments;

FIG. 6A-6C show a sensor having redundant drive and sense windings;

FIG. 6D shows a rendering of a sensor having redundant drive windings;

FIG. 6E shows a cross section of a sensor having durability enhancingpillars redundant drive winding and redundant sense elements;

FIG. 7A illustrates a flow diagram for a method of monitoring thecondition of a component under test;

FIG. 7B shows results obtained from a sensor with durability enhancingpillars that is mounted near a bolt hole under a fatigue test;

FIG. 8 is a block diagram of a system for measuring the transimpedanceof the sensor using a reference transformer for calibration;

FIG. 9A and 9B show systems for monitoring a sensor network using animpedance instrument and serially connected multiplexing units; and

FIG. 9C is a block diagram of a multiplexing unit according to someembodiments.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that an improved sensingtechnology is needed to provide reliable detection of defects nearfastener holes of a component. The sensor may be permanently mountednear the fastener hole to provide NDT measurements for the detection ofdefects near the fastener hole. In some embodiments, the sensor issubstantially planar and is pressed to the component by the fastener,washer, or another component.

The inventors have recognized and appreciated that a sensor in thisconfiguration may be subject to fretting and other forms of damage.Sensors are disclosed that include durability enhancing pillars whichextend to provide load bearing support for the sensor. The pillarsreduce the load burden on the drive winding and sense elements whichincreases the average life expectancy of the sensor. In someembodiments, the sensors are constructed of multiple substrate layersupon which drive windings, sensing elements (also referred to as sensingwindings), pillar elements and the like are formed. Pillars may beformed by providing pillar elements at the same location in multiplelayers such that a pillar is formed from the stack of pillar elements.This configuration allows the pillars to carry mechanical load, reducingthe mechanical load carried by the active elements of the sensor (e.g.,the drive windings, sense elements, and flux cancellation leads).Sufficient cumulative pillar area may be achieved by included numerouspillars over the sensor topology to provide sufficient surface area tocarry the majority of the mechanical load seen by the sensor. The numberof pillars, their shape and position may be selected to have a lowinterference with the functioning of the sensor while meeting otherdesign requirements such as providing sufficient load bearing capacity.

The sensor may be provided with one or more redundant drive windings orone or more redundant sense elements (collectively “redundantelements”). The redundant elements are configured within the sensortopology to extend the life expectancy of the sensor. In someembodiments, the redundant elements are formed on a different surface orsubstrate layer than the winding or element which the redundant elementduplicates. Each redundant drive winding or sense element may belaterally offset with respect to the winding or element which itduplicates such that when the sensor is under load the load is boreprimarily by the pillars and not the windings. Furthermore, offsettingof redundant drive windings radially from the whole enables the outerwinding to survive even if the inner winding fails. Each redundant drivewinding and sense element may be connected to separate electronics suchthat the redundant elements may be measured independently of theelements they duplicate. In some embodiments, a multiplexer is used toseparately measure each sense element response. In one such embodiment,at least two multiplexing units are included to record data fromindependent, redundant sensors.

The sensor may be excited and the response measured using an instrumenthaving an impedance analyzer. In some embodiments the impedance analyzermay use a reference transformer to recalibrate the sensor. Recalibrationmay be used to correct for contributions to the measurement that are dueto the measurement system and not the component under test. For example,drift of the system due to temperature variation may be corrected forusing a reference transformer.

Under many practical scenarios, several embedded or mounted sensors maybe monitored by the same instrumentation. Conventionally a parallelarchitecture has been used to connect these sensors to the impedanceinstrument. The inventors have recognized that in many of these sensorapplications data rate is not a critical factor as the sensors arestationary (not scanning) and the material properties are changing onlyslowly over time. A serial topology for monitoring the sensors isprovided that drastically reduces the cost and setup time associatedwith such a network of sensors. The measurement instrumentation isconnected to a set of series connected multiplexing units. Eachmultiplexing unit has a respective sensor attached thereto. Theinstrument may provide an excitation signal and control signals. Themultiplexing units take turns in accordance with the control signalproviding the excitation signal to their respective sensor's drivewinding and returning the sense element response to the instrument forcharacterization. If the sensor has multiple sense elements, themultiplexing unit may sequence the return of each sense element responseto the instrument for measurement. In another embodiment, the impedancemeasurement electronics may be miniaturized and located locally at eachsensor location (node). In one such embodiment the impedance response isconverted to an optical signal and transmitted optically. In thisconfiguration higher data acquisition rates can be provided to supportsuch applications such as torque and load monitoring.

Having discussed some embodiments generally, attention is now turned tothe drawings. FIG. 1A is a block diagram of a system 100 for monitoringthe condition of a component under test 130. System 100 includes aninstrument and data logger unit 110 (instrument 110) and a sensor 120.Instrument 110 is configured to provide excitation signals 121 to sensor120 and measure the resulting response signals 123 of sensor 120. Sensor120 may be positioned proximal to (e.g., by mounting) or embedded withina component under test 130 such that response signals 123 may beprocessed to estimate properties of component 130.

Instrument 110 may include a processor 111, a user interface 113, memory115, an impedance analyzer 117, an input/output (I/O) unit 119, and anetwork interface 121. While instrument 110 is drawn as a single block,it should be appreciated that instrument 110 may be realized as a single“box”; multiple, operably-connected “boxes”, or in any other suitableway. For example, in some embodiments it may be desired to providecertain components of instrument 110 as proximal to sensor 120 aspractical, while other components of instrument 110 may be located atgreater distance from sensor 120. Though, the configuration of thecomponents of instrument 110 with respect to sensor 120 and component130 is not critical to the invention and any suitable configuration maybe used.

Processor 111 may be configured to control instrument 110 and may beoperatively connected to memory 115. Processor 111 may be any suitableprocessing device such as for example and not limitation, a centralprocessing unit (CPU), digital signal processor (DSP), controller,addressable controller, general or special purpose microprocessor,microcontroller, addressable microprocessor, programmable processor,programmable controller, dedicated processor, dedicated controller, orany suitable processing device. In some embodiments, processor 111comprises one or more processors, for example, processor 111 may havemultiple cores and/or be comprised of multiple microchips.

Memory 115 may be integrated into processor 111 and/or may include“off-chip” memory that may be accessible to processor 111, for example,via a memory bus (not shown). Memory 115 may store software modules thatwhen executed by processor 111 perform a desired functions. Memory 115may be any suitable type of computer-readable storage medium such as,for example and not limitation, RAM, a nanotechnology-based memory, oneor more floppy disks, compact disks, optical disks, volatile andnon-volatile memory devices, magnetic tapes, flash memories, hard diskdrive, circuit configurations in Field Programmable Gate Arrays (FPGA),or other semiconductor devices, or other tangible, non-transientcomputer storage medium.

Instrument 110 provides excitation signals for sensor 120 and measuresthe response signal from sensor 120 using impedance analyzer 117. Thesensor transimpedance may be measured using one or more excitationsignals at on one or more sense elements of sensor 120. In someembodiments, time harmonic sinusoidal signals of a prescribed frequencymay be used to excite the drive winding of sensor 120. Though anysuitable excitation signal may be used. Instrument 110 may process thetransimpedance data to estimate one or more properties of componentunder test 130. Methods and apparatus for processing transimpedance datato estimate material properties are disclosed, for example, in U.S. Pat.Nos. 7,696,748 and 7,467,057 which are herby incorporated by referencein their entirety.

Memory 115 of instrument 110 may store computer executable softwaremodules that contain computer executable instructions. These modules maybe read for execution by processor 111. Though, this is just anillustrative embodiment and other storage locations and execution meansare possible. In some embodiments, aspects of impedance analyzer 117 maybe implemented as computer executable modules. Though, impedanceanalyzer 117 may be implemented using any suitable combination ofhardware and/or software.

Instrument 110 may include an input/output (I/O) unit 119. I/O 119comprises any suitable hardware and software for interfacing withinstrument 110. For example, I/O 119 may include a user interface 113and a network interface 121. I/O 119 may also comprise components forinterfacing with sensor 120.

User interface 113 may include devices for interacting with a user.These devices may include, by way of example and not limitation, keypad,pointing device, camera, display, audio input and audio output.

Network interface 121 may be any suitable combination of hardware andsoftware configured to communicate over a network. For example, networkinterface 121 may be implemented as a network interface driver and anetwork interface card (NIC). The network interface driver may beconfigured to receive instructions from other components of instrument110 to perform operations with the NIC. The NIC provides a wired and/orwireless connection to the network. Wired connections may use metalwires, optical fibers with appropriate interface electronics, or anyother suitable wired connection technology or combinations oftechnologies. Networks may be in a loop with sensor modes hung off saidloop, or with individual direct connections to a central dataacquisition unit. The NIC is configured to generate and receive signalsfor communication over the network. In some embodiments, instrument 110is distributed among a plurality of networked computing devices. Eachcomputing device may have a network interface for communicating withother computing devices forming instrument 110.

As discussed above instrument 110 may be operably connected to sensor120 through I/O 119. Sensor 120 may be any suitable type of sensingtechnology, for example, an eddy-current sensor. Sensor 120 may includeone or more drive windings and one or more sense element. If sensor 120has more than one drive winding the windings may be redundant and/orprovide different spatial modes of the magnetic field for interrogationof component 130.

FIG. 1B illustrates how sensor 120 may be adapted to inspect componentunder test 130. In the illustrated embodiment, component 130 comprisesmaterials 130A and 130B that are held together by a fastening hardware.As illustrated, the fastening hardware comprises a bolt 131 which passesthrough a fastener hole in materials 130 and 130B, a nut 133 and washers132 and 135. Sensor 120 may have a hole through which bolt 131 passesthrough. Sensor 120 may be pressed to component 130 by washer 132. Toprevent shear loads from being transferred while securing the fastener,washer 132 may be prevented from rotating during mounting by using atab, a dual washer configuration that anchors the washer from rotatingusing a second hole, or in any suitable way. In some embodiments sensor120 may be mounted under washer 135 or sensors may be mounted under bothwasher 132 and 135. It should be appreciated that other configurationsare possible and the illustration of mounting sensor 120 as shown anddescribed with connection to FIG. 1B is merely illustrative. Sensor 120may be mounted, embedded or proximally placed to component 130 in anysuitable way.

Aspects of some embodiments of sensor 120 are now discussed withreference to FIGS. 2A-7B. FIG. 2A shows the active area of a sensor 200according to some embodiments. Sensor 200 may be used as an embodimentof sensor 120 in FIG. 1A and may be configured to inspect a componentunder test as shown in FIG. 1B. Though, sensor 200 may be used in anysuitable configuration. As shown in FIG. 2A, sensor 200 includes a drivewinding 203, a first sense element 207 and a second sense element 201.Drive winding 203 and sense elements 201 and 207 may be formed fromhighly conducting materials such a metal or metal alloy (e.g.,electroplated copper). Drive winding 203 and sense elements 201 and 207may extend to a connector (not shown) for interfacing sensor 200 with aninstrument (e.g., instrument 110, FIG. 1A). Though, any suitableconnection may be used for interfacing with sensor 200.

Sensor 200 is also provided with a plurality of durability enhancingpillars 209. Pillars 209 may be made of any suitable material thatprovides rigidity to sensor 200 in the vertical direction (into and outof the page) and to substantially bear a load placed on sensor 200 whenmounted for measurement of a component under test. This relieves thedrive and sense elements from bearing this load and reduces thelikelihood that these active elements will be damaged. In someembodiments, durability enhancing pillars 209 are formed of the samematerial as drive winding 203 and sense elements 201 and 207. Forexample, pillars 209 may be formed from pillar elements etched fromelectroplated copper in the same manufacturing step in which drivewinding 203 and sense elements 201 and 207 are formed. Pillars may bearranged in patterns around drive winding 203 and sense elements 201 and207. Design considerations in the size and density of the pillarsinclude, the amount of force the mounted sensor will experience, thelife requirements for the sensor, the extent to which the presence ofthe pillars will be modeled in interpreting the sensor response, and thesensitivity to flaws required of the sensor.

Drive winding 203, sense elements 201 and 207 and pillars 209 may beformed on and/or embedded within a low-conductivity substrate 210 thatmaintains the substantially planar sensor geometry. The boundary of thesubstrate is demarcated by dotted line 211. Flexible substrate materialsmay be used so that the sensor may be conformed to a flat or curvedsurface. Though, more rigid materials may be used for a planarconfiguration of the sensor or for a curved configuration of the sensor.In some embodiments, substrate 210 is provided by one or more layers ofpoly(4,4′-oxydiphenylene-pyromellitimide) (“Kapton” available from E. I.du Pont de Nemours and Company). Multiple layers may be held together bya suitable adhesive. The active elements may be embedded between thelayers. Though, any suitable substrate material and configuration may beused.

Sensor 200 may be provided with a hole 212 in substrate 210 throughwhich a fastener may pass. Sensor 200 may be mounted such that thefastener presses sensor 200 against the component under test. Sensor 200may then be used for detection of flaws near the fastener hole of thecomponent under test. Drive winding 203 is shown having as substantiallycircular portion around hole 212 such that a current may be providedaround the fastener hole of the component and produce a magnetic fieldwithin the component proximate to the fastener hole. Sense elements 201and 207 may be positioned to couple to the magnetic field passingthrough areas of the component under test that are disposed todeveloping flaws (e.g., cracks). Such areas may be identified a priori,for example, by simulating the stresses on the component under test viasoftware of fatigue tests, by analysis of failed components, or in anysuitable way. It should be appreciated that while the fastener holeshown in FIG. 2A is circular as are the drive 203 and surrounding senseelements 201 and 207, and suitable drive winding and sense element shapemay be used. A suitable geometry may be dictated by the specificapplication for which sensor 200 is being used.

Reference line 213 of FIG. 2A indicates the location of a cross-section220 of sensor 200 shown in FIG. 2B. In the embodiment illustrated inFIG. 2B, sensor 200 comprises substrate layers 221, 223, and 225. Drivewinding 203, sense winding 207 and pillar 209 may be formed, forexample, by electroplated copper on the surface of one or more substratelayers. The electroplated copper may be etched to form the specificdrive winding, sense elements and pillars required by a particularsensor design. For example, substrate layer 223 may have electroplatedcopper on both side 222 and 224. Drive winding 203, sense element 207and pillar element 209A may be etched on surface 222 and pillar element209B may be etched on surface 224. Note that pillar 209, which is formedby pillar elements 209A and 209B, has a vertical dimension 233 that isgreater than the vertical dimension 235 the drive and sense windings.Substrate layers 221 and 225 may be provided to provide furtherprotection for the drive winding and sense elements.

In this embodiment the strength of the pillar is enhanced by verticalaligning (i.e., in a direction perpendicular to the sensors surface 226)pillar elements 209A and 209B. The lateral dimension 231 of the pillarelements 209A and 209B may be chosen with consideration to the accuracyof alignment that can be achieved between these element in themanufacturing process. It should be appreciated that the lateraldimension 231 of pillar elements 209A and 209B may be the same ordifferent. Similarly, the vertical dimension of the pillar elements 209Aand 209B may be different. For example, surfaces 222 and 224 may beelectroplated with different weights of copper. While pillar 209 isshown to be formed from two pillar elements, more or less elements maybe used provided the pillar is able to reduce the load experienced bythe drive winding and sense elements if a load is applied to surfaces226 and 228 of sensor 200.

A suitable adhesive (not shown for clarity) may be used between layers221, 223 and 225 to hold the layers together with the proper alignment.The adhesive and substrate layers 221, 223, and 225 may provide lessrigidity than the pillar elements 209A and 209B. Accordingly, when thesensor is mounted on a component for test and receives a force uponsurfaces 226 and 228 the adhesive and substrate layers may compresswhile the pillar elements substantially bear the load. While specificmanufacturing processes used for sensors according to some embodimentshave been referred to with reference to FIG. 2B, it should beappreciated that the process used to form sensor 200 is not critical tothe invention and any suitable manufacturing process may be used.

Turning now to FIG. 3A, a sensor 300 is shown. Sensor 300 may be used asan embodiment of sensor 120 in FIG. 1A. Though, sensor 200 may be usedin any suitable configuration. Sensor 300 includes a active area 301, asubstrate 302, drive leads 303, sense leads 305, and a connector area307. The length of drive leads 303 and sense leads 307 may be determinedby the particular application or any suitable way. Any suitableconnector 307 may be used for interfacing with sensor 300.

Turning now to FIG. 3B, a detail of the active area 301 is shown. Theactive area includes a drive winding 312 which has a substantiallycircular loop consisting of one turn connected to drive leads 303.Active area 301 also includes three sense elements in the embodimentshown. Sense elements 311 and 303 are located at the “top” and “bottom”of the sensor's active area (relative to the drawing orientation). Whensensor 300 is mounted, these sense elements may couple to a portion ofthe magnetic field generated by a current through drive winding 312 thatpasses through a region of the component under test where cracks orother flaws are expected to develop. Sense element 313 in turn may bepositioned on the component under test at a location that is not likelyto develop flaws. The response from sense element 313 may be used toprovide a reference point that may be used for comparison for theresponses measured on sense element 311 and sense element 321.

Sensor 300 may also include flux cancellation leads 317 for each of thesense elements 303, 311 and 313. Flux cancellation 317 follows the pathof the associated sense element lead up to the point of the senseelement area. Because of the proximity of the flux cancellation lead tothe actual sense element leads, the flux through sense element lead canbe approximated by the flux through the flux cancellation lead.Accordingly, the component of the sensed response attributed to thesensor leads can be corrected for (e.g., by instrument 110, FIG. 1A)such that the remaining sensed response may be attributed tocontributions from the region of interest. The use and design of fluxcancellation leads is detailed in U.S. Pat. Nos. 6,657,429 and 7,049,811which are hereby incorporated by reference.

FIGS. 3C and 3D show rendered images 330 and 340, respectively, of how asensor 331 may be mounted and used in a fastener application. As shownin rendering 330 a fastener bolt 335 is provided through a hole incomponent under test 333 and through a hole in sensor 331. A fastenercap 337 (e.g., a nut and washer) can then be applied and secured asshown in rendering 340 (FIG. 3D). The tightened fastener producespressure against sensor 331 which is bore primarily by the pillars 341of sensor 300. The pillars under the fastener cap will substantiallybear the load on the sensor. The remaining pillars that are outside theradius of the fastener cap will not be substantially load bearing.Though, they provide the flexibility to use a larger surface areafastener cap. It should be appreciated that this is merely oneillustrative use of a sensor with pillars; the invention is not solimited.

FIG. 4A illustrates a sensor 400 with a “near” drive winding 403 and a“far” drive winding 405. Sensor 400 may be used as a specific embodimentof sensor 120 shown in FIG. 1. Though, sensor 200 may be used in anysuitable configuration. Sensor 400 may include one or more senseelements such as sense elements 401 and 407. While drive winding 405appears to cross drive winding 403 and sense elements 401 and 407 ashort circuit is avoided because drive winding 405 is formed in adifferent plane than drive winding 403 or sense elements 401 and 407.Sensor 400 is also provided with pillars 409 in the active area of thesensor. Sensor 400 includes a substrate 412 with boundary 411. As forsensors 200 (FIG. 2A), 300 (FIGS. 3A-3B) and 331 (FIGS. 3C-3D) sensor400 is illustrated with a hole in the center of the active area throughwhich a fastener may pass if sensor 400 is to be mounted near a fastenerhole of a component under test. As for the above sensors, thisconfiguration is illustrative and sensor 400 may be adapted for anysuitable application. The pillars 409 are provided in short contactpoints distributed around the sensors active area such that the sensorresponse is not substantially affected by the presence of the pillars409 because the effect of pillar 409 is insubstantial on the sensorresponse, sensor 400 can be used to detect defects of interest incomponents under test. Drive winding 403 and drive winding 405 arepositioned radially inside and outside, respectively, of both senseelements 401 and 407. This configuration permits magnetic fields begenerated and monitored in different regions of the component undertest. When sensor 400 is mounted near a fastener hole, for example, theinner drive winding 403 produces a magnetic field that penetrates aregion of the component under test most proximal to the fastener hole.The outer drive winding 405 produces a magnetic field that penetrates aregion of the component under test further away from the fastener hole,i.e., at larger radius. This configuration permits detection of cracksor other defects that originate near the fastener hole wall or furtheraway from the fastener hole wall.

Line 413 on FIG. 4A indicates the location of the cross-section 420 ofsensor 400 shown in FIG. 4B. Cross-section 420 at line 413 illustratedin FIG. 4B is an embodiment of sensor 400 wherein the substrate isformed from substrate layers 421, 423 and 425 and from a joiningadhesive there-between. Sensor 400 may be manufactured in ways describedwith connection with FIG. 2B or in any suitable way. Cross-section 420shown in FIG. 4B is identical to cross-section 220 of FIG. 2B exceptthat cross-section 420 includes the addition of drive winding 405 whichis printed in a different plane (here between substrate layers 423 and425) than drive winding 403.

FIG. 5 shows an embodiment 500 of yet another sensor, sensor 500. Notethat in FIG. 5, the edges of each trace are drawn; the areas are not“filled in” as in the preceding sensor drawings. Sensor 500 includes aninner drive winding 507 and outer drive winding 511 connected in seriessuch that the current on the inner drive winding is in the samedirection circumferentially as the current on the outer drive winding511. In order to achieve this configuration, a via 513 is provided toconnect the inner winding 507 to outer winding 511. A second via 514 isprovided to permit the drive winding leads 512 to be in the same plane.Inner drive winding 507 and outer drive winding 511 are printed indifferent layers of the substrate of sensor 500.

Sensor 500 also includes sense elements 503, 504 and 512. In theillustrated embodiment, sense elements 503 are printed in the samematerial layer as the inner drive winding 507, though any suitableconfiguration may be used. Each sense winding may have a fluxcancellation lead such as flux cancellation lead 501 for sense element503. Because the central drive winding and the outer drive winding havecurrents traveling in the same direction, it is possible to detectsurface breaking and buried cracks with low to mid frequency eddycurrent signals.

FIGS. 6A, 6B and 6C show portions of yet another sensor (sensor 600).Specifically, FIG. 6A shows a layer 610 of sensor 600 and FIG. 6B showsa layer 620. FIG. 6C shows the layer 610 and layer 620 superimposed uponone another to form sensor 600. That is FIG. 6C shows the layers 610 and620 as they would be stacked on top of one another to form sensor 600.It can be seen in FIG. 6A that the layer 610 has a drive winding 611three sense windings, 614, 613 and 612, and a plurality of pillarelements 615 which are distributed around the entire active area oflayer 610. FIG. 6 b shows layer 620 which has a drive winding 621, threesense winding 622, 623 and 624, and partial pillars 625 that aredistributed around the entire active area of layer 620. A closecomparison of layer 610 and layer 620 reveals that the drive winding 611has a circular portion with a slightly smaller radius than the circularportion of drive winding 621. Similarly, the sense elements are formedas arcs at slightly different radii than the sense elements 622, 623,and 624 in FIG. 6B. The pillar elements 615 and 625, however, arealigned with one another such that they form pillars 635.

As the drive winding 621 produces a magnetic field that may be used toprovide sensitivity to substantially the same defects as drive winding611 these drive windings are said to form a redundant pair. If one drivewinding fails (e.g., by a crack therein that prevents current fromtraveling), substantially the same magnetic fields may be produced in acomponent under test. Similarly, sense elements 612 and 622 for aredundant pair as do sense elements 613 and 623, and sense elements 614and 624.

Because the redundant pairs are laterally offset from one another whilepillars 635 are not the pressured applied to sensor 600 when used in anapplication is bore primarily by pillars 635 rather than the senseelements or drive windings. If during use however, a drive winding orsense elements fails, the redundant windings and sense elements provideassurance that the component under test can continue to be monitored.

FIG. 6D shows a rendering 640 of the area of sensor 600 where the twodrive windings are connected to the leads. It can be seen from rendering640 that the drive windings may be formed in different layers and are ofslightly different radii than one another.

Rendering 640 also illustrates a third layer not previously shown thatincludes only pillar elements. As illustrated in rendering 640 thesepillar elements are formed as a layer between the layers of the firstand second drive windings. Thus, the complete pillar is formed by thepillar elements in the layer with the first drive winding the pillarelements in the layer with the second drive winding and the pillarelements in an intermediate layer without any drive windings.

FIG. 6E shows a radial cross-section 650 of sensor 600 in the activearea according to some embodiments. In the embodiments shown incross-section 650 only layers 610 and 620 shown in FIGS. 6A and 6B,respectively, are shown. Note from cross-section 650 that the drivewindings are printed in different layers and are slightly offset fromone another as are the sense windings. Again it can be seen from FIG. 6Ethat the pillars 650 are formed by the pillar elements 615 and 625 whichare vertically aligned with one another. The size of pillars 635 and thewidth of the sense elements may be chosen such that the manufacturingprocess used to create the sensor ensure that the pillar elements areformed with sufficient accuracy on top of one another when combined intothe sensor. Similarly, the sense and sense elements and drive elementsshould be offset from one another a sufficient amount such that there islimited or no overlap vertically between the layers. Though, the drivewindings, sense elements, and pillars may be formed in any suitable way.

Turning now to FIG. 7A, a method 700 is shown for monitoring a componentwith a sensor.

At step 701 the sensor is mounted to the component. In some embodiments,the sensor is an eddy-current sensor and may have features discussedabove and described with reference to the drawings. For example, thesensor may be mounted at a fastener hole location on the component inways similar to those described with reference to FIG. 1B. In someembodiments, the component may be a fuel tank.

At step 703 the drive winding of the sensor is excited by a electriccurrent. The excitation signal may have any suitable waveform. Forexample, a sinusoidal signal of a selected frequency may be used as theexcitation signal. The excitation signal may be generated and coupled tothe drive winding in any suitable way.

At step 705 the response of the sensor's sense element is measured. Insome embodiments, the terminal voltage of the sense element is measured.The measurement may be performed in any suitable way. In someembodiments, the response of multiple sense elements is measured at step705.

As indicated by line 704, in some embodiments steps 703 and 705 arerepeated one or more times. In each repetition of steps 703 and 705 thesame or different excitation signals may be used. For example, theresponse may be measured for exciting the drive winding at a differentfrequency. Once the desired number of repetitions are made, method 700may continue on to step 707.

At step 707 the damage condition of the component is estimated based onthe excitation signal and the sense element response. The damagecondition may be determined in any suitable way. If responses frommultiple excitation signals and/or multiple sense elements wereobtained, the measurements may be used alone, in combination, or both toestimate the condition of the component.

Method 700 may end after step 707 if the method is performed solely formonitoring the component. Method 700 may then be periodically repeatedto continue to monitor the component and/or repeated for differentexcitation signals (e.g., different frequencies). Of course mountingstep 701 may be bypassed unless it is determined that the sensor shouldbe replaced or is not properly mounted.

Optionally, method 700 may include steps 709 and 711. At step 709 it isdetermined whether a repair or replace action is required. That is, adetermination of whether the component under test needs to be repairedor replaced. In some embodiments, the determination is made based on theamount of damage. For example, in embodiments where the damage isestimate by a crack size a determination may be made whether the crackis large enough to warrant repair or replacement of the component.

If it is determined at step 709 not to perform a repair or replaceaction, method 700 ends. If, however, it is determined at step 709 toperform a repair or replace action at step 709, method 700 continues tostep 711. At step 711 the component is repaired or replaced. Forexample, a fastener hole may be oversized to remove a crack growing atits surface. After step 711 method 700 ends. Method 700 may be repeatedas discussed above.

A fatigue test performed in accordance with method similar to method700. Sensor 500 (FIG. 5) was used for the test. (Note that this test wasperformed to demonstrate sensor durability and to relate sensor responseto crack sizes. Crack size estimates were not made.) A fatigue specimenwas fabricated from annealed Ti-6Al-4V. A setup similar to that shown inFIG. 1B was used, however, the component under test was a single part.Also, rather than using washers as shown in FIG. 1B, the sensor wassandwiched between the fatigue specimen and a protective Ti-6Al-4V coverplate. The function of the cover plate is to prevent shear loads frombeing transferred to the sensor as the fastener and nut were torqued tospecific levels. The specimen was loaded into the hydraulic grips of thetest machine and fatigued under tension.

In a first test a specimen without starter electrical dischargemachining (EDM) notches was fatigued under constant amplitude loadingfor 20,000 cycles. No cracks initiated. The same sensor was transferredto a second fatigue specimen that contained an 0.016 in. EDM notch.Constant amplitude fatigue loads were applied until cracks initiated andstarted to grow from the EDM notch. After 997 cycles, the mean load andload amplitude were then decreased to maintain an R-ratio of 0.1 andcycling continued to extend the cracks until they extended past thesensing element (2,252 total cycles). At the end of the second fatiguetest, the sensor was still completely functional.

FIG. 7B shows the progression of the sensor response on the channelwhich detected the initiation and growth of the fatigue crack at 845,1067, 1,513 and 2,252 cycles. The left hand column shows a simplifiedillustration of sensor 500. Box 721 illustrates the location of detailedarea 723. Detailed area 723 shows the location of EDM notch 725. AFASTRAN simulation was used to estimate the size of the cracksthroughout the fatigue test. The FASTRAN crack estimates 729 are shownin detailed area 723 to illustrate the length of the cracks at variouscycle counts throughout the test. Curve 727 shows the normalizedresponse of the channel 2 sense element. The final crack sizes weremeasured using a low-power stereo microscope. For this test the numberof cycles at crack initiation was assumed to occur when the sensorresponse began to rise. This assumption is based on previous experience.

Attention is now turned to FIG. 8 which shows a system 800 for using areference transformer 810 to calibrate the response of a sensor 820. Theinventors have recognized and appreciated that in embedded sensorapplications it is difficult to use conventional sensor calibrationtechniques such as air or reference part calibration. Such conventionaltechniques assume that the sensor can be placed in a known environmentfor calibration. Air calibration presupposes that the sensor may belocated with nothing but air within its volume of sensitivity. Referencepart calibration presupposes the ability to place the sensor proximal toa reference part. These techniques are impractical once a sensor hasbeen mounted on a component to be tested. Without calibration,systematic error may drastically reduce the accuracy and reliability ofsensor measurements. Calibration accounts for factors that effect themeasurement that are not of interest, for example, the effect oftemperature on the electronics and other changes in environment that donot wish to be monitored.

System 800 includes impedance analyzer 830, reference transformer 810and sensor 820. Impedance analyzer 830 may be controlled by processor801 which may be configured to execute code stored on memory 803.Processor 801 and memory 803 may be similar to processor 111 and memory115, respectively, discussed above in connection with FIG. 1A. System800 may be controlled by an operator through user interface 805 or inany suitable way.

Impedance analyzer 830 is configured to provide a drive signal 835 toreference drive 809 or sensor drive 807. While establishing thereference transimpedance measurements, reference drive 809 provides thedrive excitation to primary 811 of reference transformer 810. In someembodiments, reference transformer 810 is an air core transformer.Though other core materials may be used. Reference transformer 810includes one or more secondary windings 815 that are connected in serieswith respective sense elements 825 of sensor 820. Additionally,reference transformer 810 may have a reference secondary winding 813connected in series with the reference element 823 of sensor 820.Though, reference element 823 may be located at other locations in thesystem. For example, reference element may be located in a housinginclosing reference transformer 810.

When drive signal 835 is provided by reference drive 809 on primary 811the transimpedance is measured in part by “FAB” unit 833. FAB unit 833provides filtering, amplification and buffering of the received signalsuch that, for example, a voltage may be measured for calibrating sensor820.

Specifically, the impedance measured by FAB unit 833 is impedance acrossreference secondary 813 connected in series with reference 823.Similarly, while reference drive 809 provides the drive signal toprimary 811 secondaries 815 which are series connected to respectivesense elements 825 of sensor 820 are measured by FAB unit 831.Multiplexer 840 cycles through each of the reference secondaries 815such that a reference value can be recorded by FAB unit 831 in eachcase.

Once reference transimpedance values have been measured by the impedanceanalyzer for the reference channel 813 and each of the secondaries 815of reference transformer these values are recorded in memory 803.Impedance analyzer 830 then instructs sensor drive 807 to provide theexcitation signal to drive winding 821 of sensor 820. As with referencetransformer 810 FAB unit 831 and FAB unit 833 are used to measure theimpedance for each of the series connected reference trend orsecondaries 815 and sensor elements 825. Multiplexer 840 facilitatesthis action by cycling through each of the channels. After the rawtransimpedance data is recorded by impedance analyzer 830 theprerecorded transimpedance data measured when the primary 811 ofreference transformer 810 was excited are loaded from memory 803 andused to correct the raw transimpedance data measured while drive winding821 of sensor 820 was excited. In this way robust transimpedance datacan be measured with high accuracy despite environmental variables suchas temperature fluctuations in the electronics of impedance analyzer830.

The inventors have recognized and appreciated that providing a serialarchitecture for monitoring the impedance of a group of mounted sensorsmay significantly reduce the setup burden of such a network as comparedto conventional techniques such as a parallel architecture or providingindividual impedance instruments for each sensor. According to someembodiments, an impedance instrument is connected to a plurality ofseries connected multiplexing units. Each multiplexing unit is in turnconnected to a respective sensor. The multiplexing units may be daisychained using standardized cables that may be manufactures quickly andat low cost. Unlike a parallel architecture custom cabling is not neededand only suitable lengths to be selected. The multiplexing units providesome of the electronics needed for impedance measurements closer to thesensors themselves which improves the integrity of the sensormeasurements that are made. Thus the overall size of the distributedgroup of sensors may be drastically increased. Rather than requiring allsensors to be within a few yards of one another, the serial architecturedescribed herein permits sensors to be distributes over hundreds ofyards.

FIG. 9A shows a system 900 for monitoring series connected sensors 930.System 900 includes an instrument and data logger unit 910 (instrument910). Instrument 910 are processor 911, memory 913, a user interface 912and an impedance analyzer 914. Instrument 910 may be similar toinstrument 110 described above in connection with FIG. 1A.

Impedance analyzer 914 communicates with the series connectedmultiplexing units 920 through interface 915. Each multiplexing unit 920is connected to a respective sensor 930. For example, multiplexing unit921 is connected to sensor 931, multiplexing unit 923 is connected tosensor 933 and multiplexing unit 925 is connected to sensor 935. Thesensors may be each be proximate to a component under test or atmultiple locations of a component.

A return path 940 from the last multiplexing unit 925 may be included toprovide signal integrity for system 900. In some embodimentsmultiplexing unit 925 may function as a termination unit or “caboose”providing proper termination for the series connected units. Though anysuitable termination may be used to provide signal integrity.

Impedance analyzer 914 provides an excitation signal and a controlsignal to the series connected multiplexing units 930. The controlsignal is used to designate which of multiplexing units 920 is to exciteits respective sensor with the drive signal and provided informationback to impedance analyzer 914. Specifically, multiplexing units 930return a reference signal that characterizes the drive signal asprovided to the activated sensor and one or more sense signals thatcharacterizes the response of a sense element to the drive signal.

When a multiplexing unit (e.g., unit 923) is identified by the controlsignal of impedance analyzer 914 to perform measurements, themultiplexing unit provides the drive signal to the respective sensor(e.g., 933). The drive signal couples to the sense elements of therespective sensor in accordance with the properties of the sensor andthe component under test. A voltage is produced on the sensor elementand received by the multiplexing unit. This voltage is returned as asense measurement to impedance analyzer 914. Similarly the drive signalmay be characterize and returned to impedance analyzer 914. In someembodiments, the drive signal is passed through a resistor connected inseries with the drive winding. The voltage across the resistor may thenbe returned to impedance analyzer 914. The resistor may be of knownvalue such that the voltage may be related to the current through thedrive winding. The combination of the sense element voltage and thecurrent through the drive winding may be used to define a transimpedancefor the sense element.

If the sensor is provided with more than one sense element themultiplexing unit may be configured to sequentially provide the senseelement voltage signal back to impedance analyzer 914 through the daisychain network.

FIG. 9B shows a system 950 similar to the system 900 shown in FIG. 9A.System 950 has interface box 915 located outside of instrument 910. Thisconfiguration may be preferred in some embodiments as it moves thehardware in the interface 915 closer to the multiplexing unit. It shouldbe appreciated that these distances could be considerable. For example,distances of 10, 100 or even 1,000 meters may be practical under thisserial configuration. In comparison, conventional parallelizedarchitecture with limit the distance between the impedance analyzer andthe sensors to just a few meters. As distances increased under theseconventional configurations the signal quality decreases.

FIG. 9C shows a multiplexing unit 924 according to some embodiments.This multiplexing unit is representative of any of the multiplexingunits 920 in FIGS. 9A and 9B. Though, it should be appreciated that themultiplexing units need not be identical. Multiplexing unit 924 may havefour serial communication channels as show in FIG. 9C. Control signal960 is passed from the proceeding MUX unit to control unit 970 of MUXunit 920. The control signal 960 may provide information about whichmultiplexing unit is to currently take sensor measurements. In someembodiments, the previously described reference transformerconfiguration for calibration of the sensor may be integrated into theMUX unit 920. The control signal may cause control unit 970 to activatethe reference drive to receive the drive signal 962 and provided onprimary of reference transformer. A reference signal may then beprovided to provide measurements of the reference value and similarlythe sense signal may be provided from each reference transformer channelof the secondaries.

Once the reference transformer transimpedance measurements have beenmade, or if no reference transformer transimpedance measurements are tobe made the control unit may trigger sensor drive to provide the drivesignal 962 to drive winding of sensor. The MUX may then provide each ofthe sense element responses to the FAB unit which then passes thesesignals on to sense signal channel 961.

When MUX unit 920 is not the selected multiplexing unit, it simplypasses these signals to and from the next multiplexing unit out on tothe network. If multiplexing unit 920 is the last multiplexing unit inthe series connected network the output terminals may be appropriatelyterminated. For example, a match load may be placed on the outputterminals, a signal may be returned back to the impedance analyzer, or acaboose channel may be placed at the end of the series connected MUX.

In some embodiments, multiplexing is to locate a miniaturized impedanceinstrument at each sensors node and a multiplexer to switch betweensensing elements. The multiplexer enables a single impedance channel tomeasure the response of each sensing element and a local referencetransformer. The impedance response is then communicated to a centraldata acquisition unit. The signal may be communicated over an opticalfiber network after conversion to an optical signal.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including as a local area network or a wide area network,such as an enterprise network or the Internet. Such networks may bebased on any suitable technology and may operate according to anysuitable protocol and may include wireless networks, wired networks orfiber optic networks.

Also, the various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, the invention may be embodied as a computer readablemedium (or multiple computer readable media) (e.g., a computer memory,one or more floppy discs, compact discs, optical discs, magnetic tapes,flash memories, circuit configurations in Field Programmable Gate Arraysor other semiconductor devices, or other tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

In this respect, it should be appreciated that one implementation of theabove-described embodiments comprises at least one computer-readablemedium encoded with a computer program (e.g., a plurality ofinstructions), which, when executed on a processor, performs some or allof the above-discussed functions of these embodiments. As used herein,the term “computer-readable medium” encompasses only a computer-readablemedium that can be considered to be a machine or a manufacture (i.e.,article of manufacture). A computer-readable medium may be, for example,a tangible medium on which computer-readable information may be encodedor stored, a storage medium on which computer-readable information maybe encoded or stored, and/or a non-transitory medium on whichcomputer-readable information may be encoded or stored. Othernon-exhaustive examples of computer-readable media include a computermemory (e.g., a ROM, a RAM, a flash memory, or other type of computermemory), a magnetic disc or tape, an optical disc, and/or other types ofcomputer-readable media that can be considered to be a machine or amanufacture.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of the present invention asdiscussed above. Additionally, it should be appreciated that accordingto one aspect of this embodiment, one or more computer programs thatwhen executed perform methods of the present invention need not resideon a single computer or processor, but may be distributed in a modularfashion amongst a number of different computers or processors toimplement various aspects of the present invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconveys relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A system for measuring an eddy-current sensorresponse, the system comprising: a reference transformer having aprimary winding and a secondary winding, wherein the secondary windingis configured to be connected in series with a sense element of theeddy-current sensor; a source for providing a drive signal to theprimary winding; an impedance analyzer configured to measure a firsttransimpedance of the series connected secondary winding and the senseelement while the primary winding is excited by the source, and furtherconfigured to measure a second transimpedance of the series connectedsecondary winding and sense element while a drive winding of the sensoris excited; and a processor configured to compute a calibratedtransimpedance measurement using the first transimpedance and the secondtransimpedance.
 2. The system of claim 1, where in the secondary windingis among a plurality of secondary windings of the reference transformer,each of which is configured to be connected in series with a respectivesense element of the eddy-current sensor.
 3. The system of claim 2,further comprising: a multiplexer electrically connected to each of theplurality of secondary windings and the impedance analyzer, wherein themultiplexer is configured to selectively pass a signal from one of theplurality of secondary windings to the impedance analyzer.
 4. The systemof claim 2, wherein the impedance analyzer is configured measure arespective first transimpedance of each of the plurality of secondarywindings and their respective sense element while the primary winding isexcited by the source, and further configured to measure a respectivesecond transimpedance of each of the plurality of secondary windings andtheir respective sense element while the drive winding of the sensor isexcited.
 5. The system of claim 1, wherein the reference transformercomprises an air core.
 6. The system of claim 1, wherein the impedanceanalyzer further comprises a conditioning unit to condition a sensesignal from the secondary winding prior to measurement of the firstimpedance by the impedance analyzer.
 7. The system of claim 6, whereinthe conditioning unit amplifies the sense signal.
 8. The system of claim1, further comprising a reference element, wherein the referencetransformer further comprises a reference secondary winding, and thereference secondary winding and the reference element connected inseries.
 9. A sensor monitoring system comprising: a plurality ofeddy-current sensors, each sensor having a drive winding and a pluralityof sense elements; a central data acquisition unit and configured togenerate a control signal; and a plurality of multiplexing units thatare each connected in series with one another and the central dataacquisition unit and are each connected to a respective sensor among theplurality of sensors, each of the plurality of multiplexing unitsconfigured to provide a excitation waveform to the drive winding of therespective sensor in response to a trigger in the control signal, tomeasure an impedance for each of the plurality of sense elements, and topass each of the plurality of sense element responses to the centraldata acquisition unit.
 10. A sensor monitoring system comprising: aplurality of eddy-current sensors, each sensor having a drive windingand a plurality of sense elements; an impedance analyzer configured togenerate a control signal and an excitation waveform, and to measure aresponse waveform; and a plurality of multiplexing units that are eachconnected in series with one another and the impedance analyzer and areeach connected to a respective sensor among the plurality of sensors,each of the plurality of multiplexing units configured to provide theexcitation waveform to the drive winding of the respective sensor inresponse to a trigger in the control signal and to sequentially passeach of the plurality of sense element responses to the impedanceanalyzer.
 12. The sensor monitoring system of claim 10, wherein theimpedance analyzer is at a first end of the series connections.
 13. Thesensor monitoring system of claim 12, wherein a last multiplexing unitamong the plurality of multiplexing units is at a second end of theseries connections, said last multiplexing unit having a matched load atan output terminal.
 14. The sensor monitoring system of claim 10,wherein a return path connects a last multiplexing unit among theplurality of multiplexing units to the impedance analyzer.
 15. Thesensor monitoring system of claim 10, wherein the plurality ofmultiplexing units are connected in series with an optical fibernetwork.
 16. The sensor monitoring system of claim 10, wherein at leastone of the plurality of multiplexing units has a reference transformer,the reference transformer comprising: a primary winding; and a pluralityof secondary windings, each secondary winding connected in series with arespective sense element among the plurality of sense elements of therespective sensor for said multiplexing unit.
 17. The sensor monitoringsystem of claim 16, wherein the impedance analyzer is configured to sendthe control signal towards the plurality of multiplexing units andmeasure first transimpedances of each of the plurality of secondarywindings and their respective sense element while the primary winding isexcited by the source, and further configured to measure a respectivesecond transimpedance of each of the plurality of secondary windings andtheir respective sense element while the drive winding of the respectivesensor is excited.
 18. The sensor monitoring system of claim 10, whereineach of the plurality of multiplexing units has a reference transformer,the reference transformer comprising: a primary winding; and a pluralityof secondary windings, each secondary winding connected in series with arespective sense element among the plurality of sense elements of therespective sensor for said multiplexing unit.
 19. The sensor monitoringsystem of claim 10, further comprising a resistor across through whichthe excitation waveform passes, wherein the impedance analyzer measuresa voltage across said resistor, relates said voltage to a drive current,and computes transimpedances based on the sense element responses andthe drive current.
 20. The sensor monitoring system of claim 10, whereina distance from at least one of the plurality of multiplexing units isgreater than 10 meters from the impedance analyzer.